Traditionally squirrel bristles, horse bristles, goat bristles, and other animal bristles are used as bristle materials for cosmetic brushes, and reportedly these animal bristles feel comfortable on the skin, or in other words offer good feel during use, and also provide good loading property and transfer property. Cosmetic brushes made of squirrel bristles are highly received by consumers as highest quality cosmetic brushes. Despite having the aforementioned advantages, however, animal bristles are natural resources and have some drawbacks such as limited supply, and accordingly cosmetic brush bristle materials made of synthetic fibers are proposed in recent years as alternatives to animal bristles.
For example, Patent Literature 1 proposes a cosmetic brush bristle material having recesses on its surface. The proposed cosmetic brush bristle material is made by mixing together 100 parts by weight of PBT and 5 to 15 parts by weight of silica, talc, silver zeolite or other inorganic powder with an average particle size of 0.5 to 1.0 μm, and then melting and spinning the mixture, with the obtained filament stretched by five to six times to cause the aforementioned inorganic powder to be embedded at the surface and form recesses. Filaments thus obtained are then bundled and cut to a specified length, after which one end of the obtained fiber bundle is soaked in alkali solution to enlarge the aforementioned recesses, while the one end is melted and reduced in weight and tapered. It is disclosed that the cosmetic brush bristle material using the aforementioned filament having recesses provides loading property and transfer property equivalent to those of animal bristles because the enlarged recesses created on the filament surface by means of alkali treatment act the same way as cuticles on animal bristles (refer to Patent Literature 1).
Patent Literature 2 proposes a brush bristle material made of PTT having irregularities on its surface. Hydrolyzing this PTT brush bristle material by soaking it in alkali treatment solution requires at least three times longer than the normal hydrolysis time of the PBT brush bristle material, and this resistance to hydrolysis reduces the tapering productivity and adds to cost. For this reason, the aforementioned brush bristle material has a tapered tip at the end, which is achieved by melting and spinning PTT and stretching the obtained filament by five to six times and then soaking approx. 10 to 20 mm on one longitudinal end of the bundle of such filaments in alkali treatment solution containing amine catalyst to treat the filaments for 1 to 2 hours at 110 to 130° C., thereby forming tapered tips at the ends of the aforementioned filaments by means of alkali treatment and also forming irregularities of 1 to 20 μm on their surface.
Additionally, the PTT filaments in Examples 1 and 2 of the cosmetic brush bristle material proposed in Patent Literature 2 have a fineness of 80 dtex and 100 dtex, respectively. It is disclosed that, by using a treatment solution constituted by sodium hydroxide, hydrolysis-promoting catalyst, penetrating agent, or oligomer solvent as the aforementioned alkali treatment solution, a brush bristle material having fine irregularities of 1 to 20 μm at intervals of 5 to 50 μm on the surface of the tapered tip can be obtained, and if the alkali treatment is followed by post-treatment using heated aqueous solution of benzyl ammonium chloride compound or naphthoquinone compound, antibacterial property can be added (refer to Patent Literature 2).